The present invention relates generally to alloys of titanium and aluminum. More particularly, it relates to the preparation of gamma alloys of titanium and aluminum which have been modified both with respect to stoichiometric ratio and with respect to chromium and niobium addition.
It is known that as aluminum is added to titanium metal in greater and greater proportions the crystal form of the resultant titanium aluminum composition changes. Small percentages of aluminum go into solid solution in titanium and the crystal form remains that of alpha titanium. At higher concentrations of aluminum (including about 25 to 35 atomic %) an intermetallic compound Ti.sub.3 Al has an ordered hexagonal crystal form called alpha-2. At still higher concentrations of aluminum (including the range of 50 to 60 atomic % aluminum) another intermetallic compound, TiAl, is formed having an ordered tetragonal crystal form called gamma.
The alloy of titanium and aluminum having a gamma crystal form, and a stoichiometric ratio of approximately one, is an intermetallic compound having a high modulus, a low density, a high thermal conductivity, favorable oxidation resistance, and good creep resistance. The gamma TiAl has the best modulus of any of the titanium alloys. Not only is the gamma TiAl modulus higher at higher temperature but the rate of decrease of the modulus with temperature increase is lower for TiAl than for the other titanium alloys. Moreover, the gamma TiAl retains a useful modulus at temperatures above those at which the other titanium alloys become useless. Alloys which are based on the TiAl intermetallic compound are attractive lightweight materials for use where high modulus is required at high temperatures and where good environmental protection is also required.
One of the characteristics of gamma TiAl which limits its actual application to such uses is a brittleness which is found to occur at room temperature. Also, the strength of the intermetallic compound at room temperature needs improvement before the gamma TiAl intermetallic compound can be exploited in structural component applications. Improvements of the TiAl intermetallic compound to enhance ductility and/or strength at room temperature are very highly desirable in order to permit use of the compositions at the higher temperature for which they are suitable.
With potential benefits of use at light weight and at high temperatures, what is most desired in the gamma TiAl compositions which are to be used is a combination of strength and ductility at room temperature. A minimum ductility of the order of one percent is acceptable for some applications of the metal composition but higher ductilities are much more desirable. A minimum room temperature strength for a composition to be generally useful is about 50 ksi or about 350 MPa. However, materials having this level of strength are of marginal utility and higher strengths are often preferred for some applications.
The stoichiometric ratio of gamma TiAl compounds can vary over a range without altering the crystal structure. The aluminum content can vary from about 50 to about 60 atom percent. The properties of gamma TiAl compositions are subject to very significant changes as a result of relatively small changes of one percent or more in the stoichiometric ratio of the titanium and aluminum ingredients. Also, the properties are similarly affected by the addition of relatively similar small amounts of ternary and quaternary elements as additives or as doping agents.
In a prior application, I disclosed that further improvements can be made in the gamma TiAl intermetallic compounds by incorporating therein a combination of additive elements so that the composition not only contains chromium as a ternary additive element but also contains niobium as a quaternary additive element.
Furthermore, I have disclosed that the composition including the quaternary additive element has a uniquely desirable combination of properties which include a desirably high ductility and a valuable oxidation resistance.
However, the methods by which this alloy could be prepared were limited. I have now discovered an improved and more economical method of preparing such an alloy.